Vacuum tube amplifier circuits are proposed in various areas. Such a vacuum tube amplifier circuit has a grid, a plate, a cathode and a heater to radiate electrons from the cathode. In general, when being heated by the heater, the electrons radiated from the cathode reach the plate. The amount of radiated electrons reaching the plate is regulated by the grid, thereby amplifying the input signal.
Various amplifiers utilizing a vacuum tube are proposed for applications of electric and electronics music instruments, audio instruments, etc. When a vacuum tube is used as a signal amplifier, it is so configured that an input signal is provided to the grid of the vacuum tube and the amplified output signal is obtained at the plate. Such a conventional vacuum tube amplifier is described in Japanese laid-open patent publication No. 2000-31754.
However, a conventional vacuum tube amplifier has a disadvantage in that it consumes a large amount of electric power due to the heater and thus is less efficient compared with transistor amplifiers or other amplifiers. Accordingly, a new type of a vacuum amplifier with improved electric power efficiency is desired by, for instance, decreasing the voltage for the heater.